1. Field of the Invention
The field of the invention relates to rheometers of the type where a magnet is maintained at a constant level by the action of a magnetic field.
2. Brief Description of the Prior Art
Viscous and viscoelastic properties of liquids, gels and polymer melts can be determined by a variety of types of rheometers. For liquids with low viscosities the most commonly used apparatus is the Ubbelohde viscometer. Coneplate viscometers conveniently give information about the shear rate dependence of viscosities of liquids. In general, rheological devices that measure viscoelastic moduli impose a periodic shear rate on the material and measure the corresponding shear stress, from which the elastic moduli can be calculated. Certain prior art rheometers operate by maintaining a magnetic sphere in levitation within a tube through the use of a magnetic coil. The tube is displaced at a constant velocity while the force required to maintain the sphere in position is measured. The advantages of such devices are that the shear rate can be varied over a very wide range and to very low values (e.g. 2.times.10.sup.-4 s.sup.-1), and that they can be used to measure elastic properties of fluids, melts and gels.